The present invention generally relates to a method for forming an oxide film of a semiconductor device, and an oxide film forming apparatus. More specifically, the present invention is directed to an oxide film forming method for a semiconductor device, capable of shortening pre-processing time for concentration measurements, and also to an oxide film forming apparatus.
In stages for manufacturing variable capacitors and the like, a silicon epitaxial layer corresponding to a thin silicon monocrystal layer is grown on a silicon substrate and a plurality of silicon epitaxial layers are stacked. In such a silicon epitaxial layer, a film thickness and a film quality constitute very important factors. Conventionally, such a characteristic check is carried out by measuring impurity concentration.
An impurity concentration measurement of epitaxial layer is performed by forming an oxide film on a surface. When mercury of a mercury probe of a measuring apparatus is made in contact with the formed oxide film, the oxide film forms a depletion layer, so that a Shottky barrier diode is produced. The characteristic value of this Shottky barrier diode is measured so as to measure the impurity concentration of the epitaxial layer.
As a result, it is required to form the oxide film as the pre-process operation for the impurity concentration measurement of the epitaxial layer. In the principle of this concentration measurement, the film thickness of this oxide film must be selected to be at least 15 angstrom, preferably approximately 20 angstrom.
Conventionally, as the method for forming the oxide film on a wafer, the wafer is boiled within a hydrogen peroxide water solution. The maximum film thickness of the oxide film formed by this conventional boiling method is limited to approximately 10 angstrom. It is practically difficult to form such a film thickness thicker than about 10 angstrom.
Therefore, in addition to the above-described manufacturing stage, the resultant oxide film is blown by nitrogen gas for approximately 1.5 hours, so that approximately 5 angstrom may be additionally formed on the above-described oxide film. However, there is a problem that usually 2 hours, approximately 3 hours in maximum are required as the time required to form such an oxide film having a total thickness of about 15 angstrom in addition to the film thickness of 10 angstrom by way of the hydrogen peroxide water solution boiling process. There are further problems that there is a lack of stability in the oxide film forming stage, and the reproducibility thereof is deteriorated. Moreover, since an overall film thickness is limited to on the order of 15 angstrom, there is another problem that it is practically difficult to form a film thickness thicker than 15 angstrom. As a consequence, this conventional oxide film forming method is not proper.
Furthermore, according to the thermal oxidation method in which a wafer is oxidized in high temperatures in the diffusion furnace, when the oxide film to be formed becomes the thin film, there is another problem that the uniformity of the thin film is deteriorated due to the air entrainment. In addition, as to the film thickness, a desirable range is not always obtained.
Under such a circumstance, the oxide film forming method and the oxide film forming apparatus by way of the gas discharge have been developed. As such conventional oxide film forming method and oxide film forming apparatus, for instance, the oxidation effects of ozone generated by the discharge are utilized (disclosed in, for example, Japanese Laid-open Patent Application No. 4-39931 opened in 1992, or No. 7-033405 opened in 1995).
These conventional techniques have introduced the principle structure such that the AC high voltage is applied to the electrodes provided in the gas so as to produce the gas discharge. Thus, ozone is finally generated.
In other words, when the AC high voltage applied to the electrodes provided in the gas is increased to produce the strong electric field, the generations of the electron avalanche are rapidly increased, and the electrolytic dissociation, or ionization is temporarily interrupted due to the shield effect by the space charges. Soon the electron avalanche is again generated. Thus, it is broughted into a small intermittent discharge, i.e., a corona discharge state. A large amount of ozone is generated by such a corona discharge, or the silent discharge such that the occurrence of corona is suppressed by providing the insulating material on the electrode surface. Thereafter, the silicon wafer is oxidized by utilizing the oxidation effect of this ozone to thereby form the oxide film.
On the other hand, when the supply of AC power is increased to further increase the strength of the electric field from the above-described condition that the corona discharge, or the silent discharge is generated, the discharge region is limited to the specific portion of the electrode surface, and also the emission strength is increased, so that the corona, or silent discharge is transferred to the arc discharge. As a result, the supplied power is consumed so as to heat the electrodes, so that this supplied power never contributes the generation of ozone.
Accordingly, the oxide film forming operation conditions by way of the ozone oxidation are set within the range defined from such conditions that the generation of the corona discharge, or the silent discharge is commenced up to such conditions that the corona or silent discharge is transferred to the arc discharge in the above-mentioned conventional structure by using the ozone oxidation effects.
However, it is difficult to form the silicon oxide film having the film thickness thicker than, or equal to 15 angstrom by the above-explained conventional structure by using the oxidation effects of ozone produced from the corona discharge, or the silent discharge. Moreover, there is a drawback that the time required for forming a predetermined thickness of the silicon oxide film becomes very long.
In addition, this conventional structure owns such problems that the stability of the oxide film forming process is low, and the reproducibility of forming such an oxide film is deteriorated.